(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming field isolation structure having channel stops associated therewith.
(2) Description of the Prior Art
As the size of semiconductor devices decrease there have been increasing problems. In semiconductor devices, particularly MOS field effect transistor devices there has become a problem in controlling transistor characteristics as the feature size of these devices go below one micrometer. The reasons for this problem which is termed "narrow channel effect" are the Bird's Beak effect in the field oxide isolation and the channel stop implant dopant diffusion encroachment into the channel region which reduces the effective channel width. The channel stop implant diffusion encroachment is caused by its process of manufacture which includes opening the desired pattern of oxidation mask to the regions to have field oxide isolation to be formed, ion implantation of the channel stop impurity therethrough and then the oxidation to form the field oxide isolation. The channel stop implant will deeply diffuse into the semiconductor substrate as the oxidation of the silicon progresses. During this oxidation, the dopant will encroach into the MOS field effect transistor channel regions. This encroachment will then raise the threshold voltage of the narrow channel device and making device control more difficult.
Workers in the field have understood these problems and have attempted to overcome them.
Nguyen et al U.S. Pat. No. 4,897,364 describes a method for overcoming channel stop impurity encroachment. They use silicon nitride sidewall structures on the patterned sidewalls before the channel stop impurity implant. They do not remove these sidewall structures until after the field oxide isolation is completed. The result of this is the wider spacing between field oxide isolation regions in the device region. Thus wasting valuable space.
Mitchell et al U.S. Pat. No. 4,829,019 also describes a method for overcoming channel stop encroachment. Their method includes the use of sidewall structures on the patterned sidewalls of the oxidation mask before the channel stop ion implantation. They also remove the sidewall structures before field oxide isolation. The problem presented by their process is illustrated in the Prior Art FIGS. 1 through 3. FIG. 1 show the semiconductor substrate 10, a silicon oxide layer 12 thereon, and the silicon nitride layer 14 thereover. A silicon oxide sidewall layer 16 has be formed by conventional anisotropic etching. The Boron, B ions are shown by arrows to be implanted at 18 into the substrate 10. The sidewall structures 16 are removed as by dip chemical etching. The problem is shown in FIG. 2 wherein there is an undercut etch at 20 having a length L.sub.U of the silicon oxide layer 12. The result of this undercutting after the oxidation step is a longer Bird's Beak length, L.sub.BB,U than is desirable as can be seen in FIG. 3.
It is therefore an object of this invention to provide a process for fabricating field oxide isolation pattern with channel stops associated therewith that do not have the problems of Bird's Beak with extra length and channel stop encroachment of the prior art.